A method of pressure welding two metal parts, in particular at forging temperature consists in bringing the two parts together and putting them into contact with each other, in heating them up to a certain temperature at their junction surface or join plane, and then in applying a forging pressure, such as a predetermined compression stress, perpendicularly to the join plane during cooling.
Pressure welding methods are known per se, and are widely used in numerous industrial fields, in particular for welding together metal tubes and rails for railway lines, for example. However, the use of such methods in some fields can give rise to problems related to the regulations that must be satisfied. This applies, in particular, to welding high pressure on-shore or off-shore pipelines for transporting oil or gas, where such regulations are extremely severe for obvious safety reasons. In general, all such regulations require a weld to be tested non-destructively after welding in order to detect any faults, and in particular to detect faults in compactness due, for example, to failure to melt in certain points of the weld, to the presence of cracks, to inclusions containing polluting substances such as slag, tungsten, copper, . . . , pockets of gas, etc. Acceptable sizes for each of such defects are specified in great detail in such regulations for determining whether a weld can be accepted or must be rejected, and for each type of fault detected, account is taken of its nature, its extent or volume, and its frequency. For example, such regulations are known by the abbreviations API 1104 in the USA, DNV 1981 for the bed of the North Sea, BS 4515 1984 for Great Britain, and under the name GAZ DE FRANCE for France.
Systems have been developed for testing welds, in particular by means of X-rays, gamma rays, or ultrasound, and these systems provide an image of the weld, which is then analyzed in order to decide whether to accept or reject the weld depending on value specified by the regulations, with the decision optionally also including mechanical tests.
However, although testing systems known in the past work successfully with welds obtained by adding filler material using a moving melt bath, e.g. arc welding, such testing systems are not effective with welds obtained by pressure welding methods. Thus, if steel is heated to a predetermined temperature close to 1100.degree. C. and then has a compression stress applied thereto, any weld defects due to a lack of bonding between the two faces to be welded are highly compressed and become plane. Experience shows that the above-mentioned testing systems based, in particular, on the existence of variations in density are suitable for detecting bulk defects, i.e. defects occupying a certain volume, but are not good at detecting the same defects after they have been compressed.
Preferred implementations of the invention solve the problem of non-destructively testing a weld made by pressure welding, in particular at forging temperature, by causing weld defects to become detectable.